Isolation of the Latent Precursor Complex in Electron-Transfer Dynamics. Intermolecular Association and Self-Exchange with Acceptor Anion Radicals

• Published in: Journal of the American Chemical Society 2003-03, Vol.125 (9), p.2559-2571
• Main Authors: Ganesan, Vellaichamy, Rosokha, Sergiy V, Kochi, Jay K
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Solution properties; Solutions
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0211611

Transient [1:1] complexes formed in the bimolecular interactions of electron acceptors (A) with their reduced anion radicals (A -•) are detected and characterized in solution for the first time. The recognition of such metastable intermediates as the heretofore elusive precursor complex (A 2 -•) in electron-transfer processes for self-exchange allows the principal parameters λ (Marcus reorganization energy) and H DA (electronic coupling element) to be experimentally determined from the optical (charge-transfer) transitions inherent to these intermolecular complexes. The satisfactory correspondence of the theoretically predicted with the experimentally observed rate constants validates these ET parameters and the Marcus−Hush−Sutin methodology for strongly coupled redox systems lying in the (Robin-Day) Class II category. Most importantly, the marked intermolecular electronic interaction (H DA) within these precursor complexes must be explicitly recognized, since it dramatically affects the electron-transfer dynamics by effectively lowering the activation barrier. As such, the numerous calculations of the reorganization energy previously obtained from various self-exchange kinetics based on λ = 4ΔG * must be reconsidered in the light of such a precursor complex, with the important result that ET rates can be substantially faster than otherwise predicted. On the basis of these studies, a new mechanistic criterion is proposed for various outer-sphere/inner-sphere ET processes based on the relative magnitudes of H DA and λ.